This application claims priority of Japan patent Application No. 2000-162623, filed on May 31, 2000.
This invention relates to an optical transmission system, its method, and an optical amplification transmission line.
In conventional optical wavelength multiplexing transmission on an optical amplification transmission line, gain wavelength characteristics of an optical amplifier is set to be flat so as to equalize transmission characteristics of each signal.
An effective core area of an optical fiber generally becomes smaller at a shorter wavelength than it is at a longer wavelength. In the future, when wavelength multiplexing optical transmission is performed in very broad transmission bands, it is impossible to ignore wavelength dependency of an effective core area of an optical fiber. That is, the more a transmission band broadens, the more a difference between effective core areas of the longer and shorter wavelength sides increases. Consequently, the difference of the deteriorations caused by nonlinear effects between the longer and shorter wavelengths cannot be disregarded.
It is therefore an object of the present invention to provide an optical amplification transmission line and an optical amplification transmission system which solve the above-described problems.
Another object of the present invention is to provide an optical amplification transmission system, its method, and an optical amplification transmission line which solve differences of transmission characteristics based on wavelength dependency of an effective core area.
An optical transmission system according to the invention is composed of an optical transmitter to output a first signal light having a first signal wavelength and a second signal light having second signal wavelength longer than the first signal wavelength, an optical transmission line to transmit the first and second signal lights output from the optical transmitter, and an optical receiver to receive the first and second signal lights output from the optical transmission line, wherein distance average optical power of the first signal light is smaller than that of the second signal light on the optical transmission line. The distance average optical power means the average optical power in a predetermined transmission distance on the optical transmission line.
With the above configuration, it is possible to equalize or almost equalize effective core areas and, thus, optical power density of the first and second signal wavelengths. Consequently, the nonlinear effects of the first and second signal lights become identical, and it is possible to equalize the respective transmission characteristics of the two signal lights.
For example, on an optical transmission line, a difference of optical powers between the first and second signal lights is reduced at one or more predetermined points while the optical power of the second signal light becomes larger than that of the first signal light except for the predetermined points.
When the optical transmission line is composed of at least one optical amplifier to optically amplify the first and second signal lights and at least one optical power difference reducer to reduce the optical power difference between the first and second signal lights, the aforementioned relation of the optical powers between the first and second signal lights can be realized. Preferably, the optical power difference reducer is composed of a means to practically equalize the optical powers of the first and second signal lights, and the optical amplifier has amplification characteristics in which gain of the second signal wavelength is larger than that of the first signal wavelength.
In addition, when the optical transmitter outputs the first and second signal lights onto the optical transmission line in such a condition that the optical power of the first signal light is lower than that of the second signal light, the aforementioned relation of the distance average optical powers between the first and second signal lights can be realized.
To put it more specifically, the optical transmitter outputs a third signal light having a third signal wavelength which is located between the first and second signal wavelengths. Distance average optical power of the third signal light is larger than that of the first signal light and smaller than that of the second signal light on the optical transmission line. The alternative is that the optical transmitter outputs a plurality of signal lights, each having a different signal wavelength located between the first and second signal wavelengths and a distance average optical power of each signal light is larger than that of an adjacent signal light having a shorter signal wavelength on the optical transmission line. When distance average optical powers between channels are controlled according to the above-mentioned method in multi-channel transmission, an effective core area and, consequently, transmission characteristics of each channel can be uniformed.
An optical transmission method according to the invention is composed of steps of outputting a first signal light having a first signal wavelength and a second signal light having a second signal wavelength longer than the first signal wavelength onto an optical transmission line, propagating the first and second signal lights in such a condition that a distance average optical power of the first signal light is smaller than that of the second signal light on the optical transmission line, and receiving the first and second signal lights output from the optical propagation line.
By using the above-steps, it is possible to equalize or almost equalize effective core areas and, thus, optical power density of the first and second signal wavelengths. Consequently, nonlinear effects of the first and second signal lights become identical, and therefore transmission characteristics of the two signal lights can be equalized.
For example, on an optical transmission line, an optical power difference between the first and second signal lights is reduced at one or more predetermined points while the optical power of the second signal light becomes larger than that of the first signal light except for the predetermined points.
To put it more specifically, the first and second signal lights are amplified at practically equal gain on the optical transmission line, and an optical power difference of the first and second signal lights is reduced at one or more predetermined points on the optical transmission line within a range that the optical power of the second signal light is larger than or equal to that of the first signal light. The alternative is that the first and second signal lights are amplified at the gain of the first signal light which is smaller than that of the second signal light on the optical transmission line, and the optical power difference of the first and second signal lights are reduced at one or more predetermined points on the optical transmission line within a range that the optical power of the second signal light is larger than or equal to that of the first signal light. By this, the optical power density of the first and second signal wavelengths can be easily equalized or almost equalized on the transmission line. Consequently, nonlinear effects of the first and second signal lights become identical, and transmission characteristics of the two signal lights can be equalized.
It is also applicable to output the first and second signal lights in a condition that the optical power of the first signal light is lower than that of the second signal light.
Furthermore, a third signal light having a third signal wavelength, which is located between the first and second signal wavelengths, is output onto the optical transmission line. The first, second and third signal lights propagate on the optical transmission line in such a condition that the distance average optical power of the third signal light is larger than that of the first signal light and smaller than that of the second signal light on the optical transmission line. The alternative is that the optical transmitter outputs a plurality of signal lights, each having a different signal wavelength located between the first and second signal wavelengths and each signal light propagates on the optical transmission line in such a condition that the distance average optical power of each signal light is larger than that of an adjacent signal light having a shorter signal wavelength. By controlling the relation between the wavelength and the distance average optical power on the optical transmission line according to the above-mentioned way, in multi-channel transmission, an effective core area and, consequently, transmission characteristics of each channel can be uniformed.
An optical transmission line according to the invention is to transmit a first signal light having a first signal wavelength and a second signal light having a second signal wavelength longer than that of the first signal wavelength and composed of one or more optical amplifiers to optically amplify the first and second signal lights at gains of the first and second wavelengths respectively in which optical power of the first signal light is smaller than that of the second signal light, and at least one optical power difference reducer disposed at predetermined intervals to reduce an optical power difference between the first and second signal lights within a range that distance average optical power of the second signal light in the predetermined interval is larger than that of the first signal light.
With this configuration, it is possible to easily equalize effective core areas and, thus, optical power density of the first and second signal wavelengths on the optical transmission line. As a result, nonlinear effects of the first and second signal lights become almost identical and transmission characteristics of the two signal lights are also equalized.
Preferably, the optical power difference reducer is composed of means to practically equalize the optical powers of the first and second signal lights, and the optical amplifier possesses amplification characteristics in which the gain of the second signal wavelength is larger than that of the first wavelength.